FIG. 1 shows a block diagram of a conventional direct-current to direct-current (DC/DC) converter 100. The DC/DC converter 100 includes a half-bridge switching circuit having two switches SW1 and SW2, a transformer circuit 102, and a rectifying circuit 104. The states of the switches SW1 and SW2 in the half-bridge switching circuit are controlled by a PWM driving signal DRV to selectively transfer an input power from a terminal VDC to the transformer circuit 102. The transformer circuit 102 includes a transformer T, an inductor LM, and a filtering circuit consisting of a capacitor CR and an inductor LR. The transformer T receives the input power via the switches SW1 and SW2, and converts the input power to output power. Moreover, a primary current is generated through the primary winding of the transformer T, and magnetically, a secondary current is generated through the secondary winding of the transformer T. The rectifier 104 rectifies the secondary current and provides the output power, indicated by the secondary current, to a load 106.
When switching on or switching off a switch, there will be switching losses. By way of example, when a switch receives a driving signal to be turned on, a voltage across the switch decreases toward zero over a first period of time, and a current flowing through the switch increases toward a certain current level over a second period of time that overlaps the first period of time. Therefore, power can be consumed during the turning on of the switch; and similarly, power can be consumed during the turning off of the switch. This kind of power loss can be referred to as “switching loss”.
In a conventional design, the driving signal DRV provided to drive the switches in the DC/DC converter 100 has a fixed frequency, and thus the switches are turned on and off at the fixed frequency whether the DC/DC converter 100 is powering a heavy load or a light load. However, it is unnecessary to switch the switches on and off if the DC/DC converter 100 provides power to a light load. Thus, under light load conditions, power is unnecessarily consumed and the power conversion efficiency is reduced due to switching losses.
Additionally, a turn-on delay exists between the time when a driving signal is generated to turn on a switch and the time when the switch is fully turned on, and a turn-off delay also exists between the time when a driving signal is generated to turn off the switch and the time when the switch is fully turned off. These delays may be caused by non-ideality of the switch and/or associated circuitry such as a driver (not shown) that drives the switch. If the DC/DC converter 100 is powering a light load, the DC/DC converter 100 may reduce an ON time, i.e., a pulse width, of the driving signal DRV to a relatively small value which is comparable to the delays. Similarly, if the DC/DC converter 100 is powering a heavy load, the DC/DC converter 100 may increase the pulse width of the driving signal DRV so that an OFF time, i.e., the time during which the driving signal DRV is logic low, is relatively small and comparable to the delays. As a result, the ON time or the OFF time may not be long enough to turn on or off the switches SW1 and SW2 properly.
Furthermore, drivers (not shown) that control the switches SW1 and SW2 may have different time delays, which causes the switches SW1 and SW2 to be on at the same time. If the switches SW1 and SW2 are on at the same time, then the power source at the terminal VDC is short-circuited to ground via the switches SW1 and SW2, and the switches SW1 and SW2 suffer a large current pulse. This may cause damage to the power source and the switches SW1 and SW2. A power converter that addresses these shortcomings would be beneficial.